A driving performance, an accelerating performance, a fuel economy etc. of an automobile are significantly influenced by a rotational speed and an output torque of an engine serving as a prime mover of the automobile. In recent days, the speed and the output torque of the engine have been allowed to be controlled independently. Therefore, attempts have been made to improve the driving performance and the fuel economy of a hybrid vehicle whose engine speed can be controlled by a motor, and a vehicle having a continuously variable transmission, utilizing such control function.
For example, Japanese Patent Laid-Open No. 2001-112155 discloses a control apparatus configured to improve the driving performance and the fuel economy of a hybrid vehicle. According to the teachings of Japanese Patent Laid-Open No. 2001-112155, an operating speed of the engine is changed along the operating line for improving the fuel economy when charging the battery. Meanwhile, when increasing a power, an operating speed of the engine is changed along the operating line for generating a larger torque while suppressing an increase in the engine speed, and then the engine speed is changed along the target output line. In turn, Japanese Patent Laid-Open No. 2005-76673 discloses a speed change controller for improving the re-accelerating performance to exit a corner. To this end, the speed change controller taught by Japanese Patent Laid-Open No. 2005-76673 is configured to calculate a required minimum engine rotational speed is calculated based on a required engine output calculated based mainly on a running resistance, and to carry out a speed change including a downshifting during passing on a curve while inhibiting an upshifting to maintain the minimum engine rotational speed. Further, PCT international publication WO2011/021634 discloses a vehicle control system configured to control characteristics of driving force, speed ratio, suspension etc. based on a command index representing driving preference of the driver that is obtained based on a synthesized acceleration of the longitudinal acceleration and lateral acceleration. The command index is increased in accordance with an increase in the synthesized acceleration. However, the command index is maintained to a current value until a satisfaction of a predetermined condition if the synthesized acceleration is decreased.
According to the teachings of Japanese Patent Laid-Open No. 2001-112155, when the drive demand is increased to accelerate the vehicle, the engine is driven away from the optimum fuel line to increase the torque to comply with the demand. However, if the control taught by Japanese Patent Laid-Open No. 2001-112155 is carried out to re-accelerate the vehicle after the required power is reduced, the torque may be consumed significantly to change the engine speed thereby deteriorating the re-accelerating performance. As described, according to the teachings of Japanese Patent Laid-Open No. 2001-112155, the operating speed of the engine is changed along the operating line for generating a larger torque when the required power is increased. That is, when the required power is decreased, the operating speed of the engine is dropped significantly to be changed along the operating line for improving the fuel economy. In this situation, if the required power is increased again to re-accelerate the vehicle, the operating speed of the engine is again raised significantly to the point on the operating line for generating a larger torque. Therefore, the torque may be consumed drastically to increase the engine speed thereby deteriorating the re-accelerating performance.
As also described, the speed change controller taught by Japanese Patent Laid-Open No. 2005-76673 is configured to improve the re-accelerating performance to exit a corner. For this purpose, the speed change controller maintains the engine speed to the required minimum engine speed when the vehicle enters into a corner or during travelling on a curve while inhibiting an upshifting. However, according to the teachings of Japanese Patent Laid-Open No. 2005-76673, the required minimum engine speed is calculated based on the running resistance. That is, although the driving force sufficient to overcome the running resistance can be generated, the driving performance or the accelerating performance may not be adjusted while reflecting a driver's intention sufficiently.
As also described, according to the teachings of PCT international publication WO2011/021634, the command index is increased when the synthesized acceleration is increased. In contrast, when the synthesized acceleration is decreased, the command index is maintained to the current value and prevented from being lowered. However, if an accelerating operation and a decelerating operation are carried out consecutively under the situation where the command index is maintained to a high level, the actual behavior of the vehicle may not be adjusted sufficiently while reflecting the driving preference of the driver represented by the command index.